Reliability assessment of structural concrete with special reference to stirrup design

Mensah, Kenneth Kwesi (2015-03)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Structural design standards based on the principles of structural reliability are gaining worldwide acceptance and are fast becoming the new basis for structural safety verification. The application of these principles to establish a standardised basis for structural design using partial factor limit states design procedures is done in the European Standard for the Basis of Structural Design EN 1990 from which it is adapted to the South African Standard Basis of Design for Building and Industrial Structures SANS 10160-1. South Africa (SA) is on the advent of adopting the European Concrete Design Standard EN 1992-1-1 (EC2) as the equivalent standard for local use. This investigation seeks to provide a transparent quantitative reliability basis for the SA’s adoption of EC2, as well as provide for its subsequent implementation under local conditions and practice. The investigation kicks-off with a critical review of the reliability framework for structural resistance. The review establishes the relationships between the key elements of the framework, shedding light on issues SA needs to consider as it adopts EC2. Important issues for SA to consider include (1) target levels of structural performance ( -values), (2) partial factors, (3) model uncertainties, and (4) quality control. Design for shear resistance was investigated in greater detail by comparison of EC2’s Variable Strut Inclination Method (VSIM) for stirrup design against alternative approaches, namely, (1) South Africa’s currently operational SANS 10100-1 procedure, and (2) the fib Model Code 2010 first Level of Approximation (LoA I) and fib LoA III, which are based on the Modified Compression Field Theory (MCFT). Unbiased capacity predictions from the MCFT-based sectional analysis Program Response-2000 (R2k) served as LoA IV best-estimate results during this assessment. Results of this investigation showed that EC2 offers higher capacity predictions in excess of 1 MPa of stirrup reinforcement, with significantly higher predictions in the range of 1 to 2 MPa. A reliability performance assessment was therefore commissioned to assess safety regimes in terms of achieved reliability across a parametric range of the amount of stirrup reinforcement (from 0.45 to 2.0 MPa). The First Order Reliability Method (FORM) was implemented as part of the reliability performance assessment of the EC2’s VSIM design procedure. The model uncertainty for shear resistance (stirrup failures) was characterised according to a database of published stirrupreinforced concrete beam shear tests. Three cases of the Model Factor for shear resistance were derived from the experimental database for alternative shear resistance prediction models; two of which formed part of basic investigations conducted using the conventionally formulated performance function, and the other was integrated as part of an independent validation procedure using R2k predictions to obtain the reliability model. Results obtained from the basic reliability model ( -values) generally indicated lower levels of reliability with an increase in stirrup reinforcement and concrete strength, compared to those estimated from the R2k-based reliability model ( -values). The disparity between and -values revealed that systematic effects affect each model’s ability to predict the expected value of true shear resistance . There is reasonable evidence to suggest that the predictions of can be improved by accounting for each model’s peculiar sensitivity to concrete strength, consequently providing more representative estimates of . However, in the interim, and -values, respectively, represent reasonable lower and upper bound estimates of the performance of EC2’s VSIM design procedure.

AFRIKAANSE OPSOMMING: Die beginsels van struktuur betroubaarheid word wêreldwyd aanvaar as basis vir struktuur ontwerp standaarde en die versekering van voldoende struktuur veiligheid. Hierdie beginsels word in die Europese Standaard Basis of Structural Design EN 1990 toegepas om gestandaardiseerde partiële faktor gebaseerde limietstaat ontwerp prosedures daar te stel, van waar dit aangepas is vir die Suid-Afrikaanse Standaard Basis of Design for Building and Industrial Structures SANS 10160-1. Suid-Afrika (SA) staan op die punt om die Europese beton ontwerp standaard EN 1992-1-1 (EC2) aan te neem as die ekwivalente standaard vir plaaslike gebruik. Hierdie ondersoek het as doel om ‘n deursigtige kwantitatiewe betroubaarheidsbasis daar te stel vir die aanneming van EC2 as SA standaard en om voorsiening te maak vir die implementering daarvan onder plaaslike toestande en –praktyk. Die ondersoek begin met ‘n kritiese oorsig van die betroubaarheidsraamwerk vir strukturele weerstand. Die oorsig stel die verhouding vas tussen sleutel elemente van die raamwerk en werp lig op aspekte wat SA moet oorweeg in die aanneming van EC2. Belangrike aspekte vir oorweging sluit (1) teiken betroubaarheidsvlakke vir struktuur gedrag ( -waardes), (2) partiele faktore, (3) model onsekerhede en (4) kwaliteitsbeheer in. Skuif weerstandsontwerp is in groter detail ondersoek deur die EC2 se Veranderbare Stut Hoek Metode (VSHM) vir skuifbeuel ontwerp te vergelyk met alternatiewe benaderings, naamlik, (1) Suid Afrika se huidig operasionele SANS 10100-1 prosedure, (2) fib Model Code 2010 se sogenaamde eerste Vlak van Benadering (VvB I) en fib VvB III, gebaseer op die Aangepaste Drukveld Teorie (ADT). Onbevooroordeelde kapasiteit voorspellings van die ADT-gebaseerde snit analise program “Response-2000 (R2k)” is in die evaluering gebruik as VvB IV bes benaderde resultate. Die ondersoek toon dat EC2 hoër kapasiteit voorspel vir skuifbeuel bewapening tot 1 MPa en beduidend hoër kapasiteite voorspel vir skuifbeuel bewapening tussen 1 en 2 MPa. ‘n Betroubaarheidsprestasie studie is vervolgens geloots om die veiligheid in terme van behaalde betroubaarheid te bepaal oor ‘n parametriese bereik van 0.45 tot 2.0 MPa skuifbeuel bewapening. Die Eerste Orde Betroubaarheids Metode (EOBM) is implementeer as deel van die betroubaarheidsprestasie beoordeling van die EC2 VSHM ontwerp prosedure. ‘n Databasis van gepubliseerde skuifbeuel-bewapende betonbalk skuiftoetse is gebruik om die model onsekerheid vir die verskillende skuifweerstandsmodelle statisties te beskryf. Drie Model Faktore is uit die eksperimentele databasis afgelei, twee waarvan gebruik is in basiese ondersoeke met die konvensioneel geformuleerde prestasie funksie en die derde as deel van ‘n onafhanklike bevestigingsprosedure gebaseer op R2k voorspellings. Resultate wat verkry is uit die basiese betroubaarheidsmodel ( -waardes) was laer (meer konserwatief), en het ook vinniger afgeneem met ‘n toename in skuifbeuel bewapening as die waardes wat uit die R2k-gebaseerde betroubaarheidsmodel ( -waardes) verkry is. Die verskil tussen β en β -waardes toon dat sistematiese effekte die vermoë van elk van die modelle beïnvloed om die verwagte waarde van die werklike skuifweerstand V te voorspel. Daar is redelike bewyse om aan te voer dat die voorspellings van V verbeter sal kan word deur elke model se unieke sensitiwiteit teenoor betonsterkte in ag te neem, om sodoende meer verteenwoordigende β waardes te verkry. Intussen verteenwoodig die β en β -waardes onderskeidelik redelike onder- en bogrens skattings vir die prestasie van EC2 se VSHM ontwerp prosedure.

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